Technical Field
The present invention is related to a method for preparing an electrode material; especially related to a method for preparing a cathode material for lithium ion batteries.
Description of Related Art
The main trend of the demands of the current electronic, informative, bio-medical apparatuses or devices is to be compact and miniaturized. In order to fulfill the demands, the batteries used in those apparatuses and devices are also expected to be smaller but still have the advantages of high is storage capacity and high discharge capacity. Lithium ion batteries have gradually drawn the attention in academic and industrial area as complying with the aforesaid requirements.
The main elements of a lithium ion battery includes an anode (LiCoO2—LiMn2O4—LiFePO4, etc), an electrolytic solution, a separation membrane, and a cathode (carbon-based material and titanium-based material). The working mechanism of a lithium ion battery is to utilize the oxidation and reduction of the lithium ion between the anode and the cathode to provide charging and discharging reaction. The chemical reaction of the charging and discharging can be summarized as the following formulas; wherein M is referred to as Co, Ni, or Mn; the reaction directs toward right side while charging and left side while discharging:Reaction at anode: LiMO2Li(1−x)MO2+xLi++xe−Reaction at cathode: C6+xLi++xe−LixC6 Net reaction: LiMO2+C6Li(1−x)MO2+LixC6 
In recent researches, several transition-metal oxides have been used as cathodes of lithium ion batteries, such as MoO2, SnO2, CoO2, CuO, FeO, Li4Ti5O12, TiO2 and etc. Among them, titanium-based materials such as TiO2 and Li4Ti5O12 have been widely studied for their properties of stable crystal structure, low volume change upon charging/discharging (less than 0.2%) and excellent life cycle (at least 1500 cycles) and their promising application as cathode materials of lithium ion batteries to replace carbon-based materials. Although the aforesaid titanium-based materials provide better safety, their drawbacks of bad conductivity and lower electrical capacity (about 180 mAh/g) are the first-priority issues to be overcome for being used as the cathodes of lithium ion batteries.
U.S. Pat. No. 6,007,945 disclosed a solid phase reaction to mix TiO2 and SnO2 at various ratios for being used as the cathodes of lithium ion batteries. The ratios of the Ti atom and Sn atom taught in the Patent are Ti:Sn=6:5 (Ti6Sn5) and 2:1 (Ti2Sn1) respectively for mixing commercial TiO2 powder and SnO2 powder. Then, the mixture was sintered at 1000° C. and crushed to an average diameter of 15 μm. After that, the powder was combined with 5 wt % of conductive carbon (ex. fine petroleum coke) and adhesives (ex. polyvinylidene fluoride) and blended evenly to form as a working electrode. The obtained working electrode was examined by using LiCoO2 as the counter electrode. The examination results showed that the reversible capacity of Ti6Sn5 and Ti2Sn1 is 1130 mA/cm3 and 1110 mA/cm3, respectively (while the density of the electrode is both 3.65 g/cm3). In addition, the initial working voltage of the LiCoO2 battery was 3.5V and maintained at 3.2V after 50 cycles.
An article of Materials Research Bulletin (46 (2011) p. 492-500) disclosed to mix homemade Li4Ti5O12 and Sn evenly by high energy ball milling to obtain a novel composite of Sn/Li4Ti5O12. The author taught to mix the quantified organic lithium and titanium dioxide evenly and sintered the same at various temperatures to obtain a composite of Li/Ti/O. According to the experiments, the Li4Ti5O12 obtained at 800° C. sintering temperature has a purer crystal phase. The Sn nanopowder was obtained by chemically mixing SnCl2.2H2O with NaBH4 solvent. Then, the Li4Ti5O12 was mixed with Sn at various ratios to obtain the Sn/Li4Ti5O12 mixture. According to the article, Li4Ti5O12 could reduce the overexpansion problem of Sn upon charging/discharging and Sn was able to provide better conductivity to Li4Ti5O12 and make the Sn/Li4Ti5O12 mixture have good cycling stability and high capacity. In the sample of Li4Ti5O12:Sn=70:30, the initial discharge capacity was 321 mAh/g and maintained at 300 mAh/g after 30 cycles.
An article of Journal Alloys and Compounds (462 (2008) p. 404-409) disclosed to deposit Sn compound to Li4Ti5O12 by chemical deposition and to obtain a Li4Ti5O12 composite coating with SnO2 after heating. First of all, SnCl2.xH2O was dissolved in ethanol for various ratios. Then, Li4Ti5O12 and NH3.H2O were sequentially added to deposit Sn on the surface of the Li4Ti5O12 material in the form of an oxide. After that, the material was sintered at 500° C. for 3 hours to obtain a Li4Ti5O12—SnO2 composite. The data showed that the method can evenly modify the surface of Li4Ti5O12 with SnO2 and provide the batteries with stable life cycle and electronic capacity. The initial discharge capacity of Li4Ti5O12 modified with 5 wt % of SnO2 on the surface thereof was 443 mAh/g and maintained at 189 mAh/g after 42 cycles.
An article of Transactions of Nonferrous Metals Society of China (20 (2010) s267-s270) taught the preparation of SnO2—Li4Ti5O12 composite by a sol-gel process. First of all, tetrabutyl titanate after quantization was dissolved in anhydrous ethanol and added to an ethanol solution containing lithium acetate. After stirring, a powder of SnO2 was added in and the mixture was kept vibrating for 2 to 3 hours by a sonicator. Then, the mixture was dried and sintered at 500° C. for 4 hours to obtain the SnO2—Li4Ti5O12 composite. The experimental results showed that the SnO2 powder was coated with a layer of amorphous Li4Ti5O12 and formed a core-shell structure. The structure controlled the volume expansion of SnO2 upon charging and discharging and thereby prevented the material from being collapsed. The composite had a 688.7 mAh/g of reversible capacity upon the initial charging and discharging, and the reversible capacity maintained at 93.4% after 60 cycles, showing that the composite was a cathode material with high capacity and long lifetime.
In light of the foregoing publications, the combination of titanium-based material and tin salts has been widely studied recently. The kind of composite not only can combine the advantages of the two materials but also can minimize the drawbacks thereof so that forms a cathode of good life cycle, high electronic capacity and high safety. However, it is also noted from the aforesaid disclosures that the fabrication of the composite is too complicated and expensive thereby is not ideal for commercialization. Improvement to overcome this matter is in need.